1. Field of the Invention
The present invention relates to a coated hard tool used as a cutting tool requiring wear resistance, or other wear resistant tool. More specifically, the present invention relates to an improvement of wear resistance and seizure resistance of the tool.
2. Description of the Background Art
Conventionally, a super hard alloy has been used for a cutting tool. Such a super hard alloy may include a WC--Co alloy to which a carbide, a nitride, a nitrided carbide or the like of Ti, Ta, Nb or the like are added, for example.
Recently, as the speed of cutting increases, a type of coated hard tool has come to be frequently used, in which a hard coating having a thickness of about 3 to about 20 .mu.m is formed by PVD (Physical Vapor Deposition) on a surface of a hard base material such as a super hard alloy, a cermet, an alumina based ceramic, a silicon nitride based ceramic or the like. A film containing a carbide, a nitride, a nitrided carbide, a nitrided boride or an oxide of an element in group 4A, 5A or 6A of the periodic table or of Al may be used as the hard coating. The hard coating deposited by the PVD method improves the wear resistance of the base material while not deteriorating the strength of the base material. Therefore, the hard coating is widely used for cutting tools which require strength, such as drills, end mills and throwaway tips (exchangeable, disposable tips) for milling.
Japanese Patent Laying-Open No. 8-127862 discloses a coated hard tool including a hard coating of multi-layered structure. FIG. 3 is a schematic cross section of a part of the coated hard tool containing such a multi-layered hard coating. The coated hard tool of FIG. 3 includes a hard coating 2' deposited on a hard base material 1'. In the hard coating 2', a plurality of layers of two or more types are repeatedly stacked periodically, with each layer having a thickness in the range of about 1 to about 100 nm. Each layer included in the hard coating 2' may include one or more compounds selected from a nitride, a carbide, a nitrided carbide and an oxide containing at least one element selected from periodic group 4A elements, periodic group 5A elements, Al, B and the like.
In the hard coating 2', adjacent layers of different types have crystal lattice constants that are slightly different from each other, and include a crystal lattice structure partially continuing at interfaces therebetween. In such a multi-layered film including crystal lattice structure partially continuing at the interfaces between the layers, considerable internal strain is generated by an elastic complementary effect between the layers, and as a result, a greater hardness than the inherent hardness of each single layer is obtained.
FIG. 4 is a schematic cross section of another example of a coated hard tool including a hard coating of multi-layered structure. The coated hard tool of FIG. 4 is similar to that of FIG. 3, except that an intermediate layer 3' is interposed between hard base material 1' and multi-layered coating 2' in FIG. 4. Intermediate layer 3' is provided to improve adhesion between hard base material 1' and multi-layered coating 2'. A nitride, a carbide, a nitrided carbide or the like of periodic group 4A element may be used as the intermediate layer 3'.
As described above, very high hardness is obtained because of interaction between layers in such a multi-layered coating 2' as shown in FIG. 3 or FIG. 4. However, unless strain matching between layers is stabilized appropriately, the coating 2' may be subjected to embrittlement because of too high an internal stress. Particularly, the internal strain increases as the thickness of multi-layered coating 2' increases. Therefore, when the coating is too thick, coating 2' may suffer from brittle fracture because of shock experienced during cutting.